Although such batteries enable high energies to be obtained, their stability is fairly low because their negative electrodes are corrodable by reaction in the alkaline electrolyte which means that the battery looses capacity during storage because the electrode is partially dissolved in the electrolyte. Continuous and considerable evolution of gaseous hydrogen also occurs, thereby causing electrolyte to ejected and sometimes even giving rise to gas pressures that burst the battery's safety valve. The magnitude of these harmful phenomena increases with increasing temperature, e.g. when batteries are stored in a hot country.
In order to avoid these phenomena, all battery manufacturers have for years been amalgamating their zinc electrodes, and sometimes also adding cadmium or lead thereto. Mercury and lead are simple and effective means for reducing the overvoltage at which hydrogen is given off, in other words for avoiding zinc corrosion, with cadmium additionally serving to improve conduction. The zinc electrode is thus constituted by a gelled mixture of amalgamated zinc powder containing lead and cadmium powder and an alkaline solution of concentrated potassium hydroxide. A current collector constituted by a metal cylinder is plunged into this electrode.
The quantities of mercury, cadmium, and lead used in each battery are small, but there exists abundant and worrying literature concerning the dangers of dispersing these elements in the environment and the very serious risk of them finding their way into the food chain. When consumers throw batteries away, the batteries suffer two types of fate: either they accumulate in tips or land fills, in which case the can of the battery retains the battery contents for a certain time, after which it corrodes and the active substances, including mercury, are leached by ground water; or else the batteries are incinerated in which case the mercury distills and escapes with the smoke into the atmosphere prior to recondensing roundabout. In either case mercury gets into the environment.
Although it is sometimes recommended that worn-out batteries containing mercury, cadmium, or lead should be recycled, this does not appear to be an ideal solution since it would inevitably lead to the creation of concentrated waste which would be even more dangerous to take care of.
Thus, looking beyond regulations which require the content of these metals in batteries to be reduced, it is essential to find ways of omitting them altogether. However, since a low content of mercury (less than 3% by weight relative to the zinc) is already not enough on its own to ensure battery conservation, several replacement solutions have already been tried.
Some of the more effective solutions have already been proposed by the present Assignee. Thus, French patent number FR-A-2 567 328 describes a method of stabilizing a zinc electrode by incorporating a stabilizer at a concentration lying in the range 0.01% to 1% by weight of zinc, said stabilizer being constituted by an organic polyfluoride compound of the ethoxyl fluoroalcohol type. In addition, in French patent application No. FR-A-2 583 580, the Assignee has proposed another compound of the alcoyl sulfide and polyethoxyl alcohol type at a concentration of 0.001% to 1% by weight. In both the above two cases, it has been observed that although the problem of zinc corrosion is solved, there is also a drop in the level of the battery discharge voltage, and that this drop increases with increasing demand, i.e. with increasing current.
The objet of the present invention is to avoid this drawback and provide a "clean" alkaline primary cell having a zinc negative electrode, in other words a battery containing no mercury, no cadmium, and no lead, but having the same performance as a battery in which the zinc is 5% amalgamated.